1. Field of the Invention
This invention relates to coaxial helicopter systems. More particularly, the present invention relates to an ultralight coaxial helicopter system.
2. Discussion of the Related Art
Coaxial helicopters were first developed, in the form of small devices used as toys and curiosities, centuries ago. The earliest attempts at designing a practical helicopter focused on coaxial rotors and dual counter-rotating arrangements. Later, what has come to be thought of as conventional helicopter designs were developed. These were single-rotor helicopters, and it was found that they needed a long tail boom having a tail rotor at the end rotating in a plane roughly perpendicular to the plane of rotation of the main rotor, in order to apply a consistent counter-reaction moment force to counteract the rotational reaction forces arising from powering the single rotor. These reaction forces tend to make the airframe of the helicopter rotate in a direction opposite that of the direction of rotation of the rotor. Without the tail rotor to provide the counter-moment, the airframe could rotate uncontrollably once airborne due to these reaction forces. This need for a tail rotor has given us the readily-recognized shape of conventional single-rotor helicopters including the long tail boom with a tail rotor at the aft end.
As mentioned, earlier it was theorized and proven that a helicopter with two counter-rotating rotors could be built such that the rotational force of one rotor counteracts the rotational reaction force of the other, leaving the helicopter body stable without the need for a perpendicularly acting tail rotor. The first controllable man-carrying helicopters were tandem-rotor designs. Tandem-rotor helicopters remain the most common dual-rotor helicopters.
Tandem-rotor helicopters such as the CH-46 Chinook aircraft manufactured by Boeing Aircraft Corp. of Seattle, Wash. have been found to be particularly useful for heavy lifting operations and otherwise where a large payload capacity is needed. Conventional tandem-rotor helicopters typically have an elongate body with a first rotor atop the front end, and a second rotor atop the rear end. The rotors can be elevationally offset so as to avoid contact with each other when rotating, or they may be separated by a sufficient distance to prevent contact. They may also be configured with a rotor indexing means which allows the rotor blades to intermesh during rotation and at the same time keeps them from coming into contact with each other. This later configuration is sometimes analogized to, and referred to as, an “eggbeater” arrangement.
Dual-rotor helicopters with coaxial rotors have also been developed. These helicopters include two counter-rotating rotors mounted on a single axis. While, as mentioned, coaxial helicopters have been known for many years, development of this type of aircraft has heretofore been limited because of complexities involved in arrangements for control of the rotor blades to give roll, pitch and yaw control. In conventional coaxial designs at least two swashplate assemblies are provided to provide collective and cyclic pitch control on both rotors. A substantially conventional swash plate is provided below a lower rotor; and a swash plate assembly incorporating two counter-rotating swashplate portions is provided between the upper and lower rotors. Associated control links, push rods, etc. are needed, all so that cyclic and collective pitch control inputs to the upper rotor can be transferred past the counter-rotating lower rotor. As is known, using this arrangement it is a daunting task to provide a reliable aircraft without unduly burdensome maintenance requirements. The control arrangements are necessarily complex, and relatively high forces must be transferred by the swashplate assemblies and control links, so they must be robust, and accordingly, heavy. This arrangement does not allow differential collective to be applied for yaw control, and so a further means for yaw control is typically provided. This can be in the form of additional collective control links and mixing arms, adding a yaw fan, or making the swash plates movable with respect to each other, etc, but additional structure (with attendant additional weight) typically is included to provide this differential collective control.
For these reasons, and others, in smaller helicopters conventional single-rotor designs, having a tail rotor for yaw control and for counteracting the tendency of the airframe to turn with respect to the rotor, predominate. Nevertheless, several successful coaxial designs have been developed, for example, by Nikolai Kamov and the Kamov design bureau of the former Soviet Union. The Kamov Company organization of Lubertsy, Moscow Region, Russia continues to successfully design and produce coaxial helicopters. Other coaxial designs exist, for example a small coaxial pilotless craft developed by the Sikorsky division of United Technologies Corporation, of Hartford Conn. An example of a control system for this latter craft is disclosed in U.S. Pat. No. 5,058,824. Another example is the XH-59 ABC technology demonstrator helicopter, also built by the Sikorsky division.
All aircraft, helicopters included, require control of attitude (including pitch, roll, and yaw), and linear motion (speed). The main rotor of a conventional single-rotor helicopter is typically configured to vary the pitch of the rotor blades cyclically and/or collectively to control pitch, roll, and lift, and therefore forward motion (or reverse, or side-to-side motion). Collective blade pitch control of the tail rotor controls yaw. The power output of the engine may also be varied, albeit within a fairly narrow operational power band, and this can affect lift and yaw.
In a conventional tandem-rotor and coaxial helicopters, these same attitude and lift controls are effected by cyclic and/or collective pitch variation of the blades of both rotors. Yaw control is by differential collective control inputs to the counter-rotating rotors, causing one to have more drag and the other less, thereby turning the aircraft about the yaw axis.
Coaxial helicopters potentially present many advantages over conventional single- and tandem-rotor helicopter designs. They can be more compact than a single-rotor design because of higher disk loading, and the fact that they have no need for a tail rotor for counter-acting the tendency of the airframe to turn around the rotor axis in reaction to the torque input to the rotor. Coaxial designs are more compact than tandem and eggbeater designs because there is no need to separate the rotors except for vertical rotor clearance. Because of said higher disk loading, coaxial designs can provide a given desired lifting force using a smaller diameter rotor set than comparable single-rotor helicopters. They typically require a smaller airframe than a comparable eggbeater or tandem-rotor helicopter. Moreover, because the rotors of a coaxial helicopter are disposed one on top of the other, and are counter-rotating, power efficiency losses due to vortex air movement adjacent the upper rotor can be at least partially recovered in increased effective airspeed and lift in the lower rotor. In other words, the upper rotor gives the air a swirl in one direction, and the lower rotor swirls it in the other, canceling out a good part of the imparted vortex air movement. Also, elimination of the tail rotor frees up the engine power otherwise diverted there. This savings has been cited as up to about 30% of total engine power output in some cases.
However as noted above, there is a trade-off for these advantages, in that providing for the control of coaxial rotor helicopters presents additional complexities and increased swashplate, linkage, and rotor weight and increased maintenance concerns. One approach to mitigating the disadvantages of a coaxial arrangement is to eliminate the need for swashplates and complex control linkages altogether. Rather than adjusting the pitch of the coaxial rotor blades, an alternative for controlling coaxial helicopters is to make the axis of rotation of the coaxial rotor set tiltable with respect to the airframe, allowing pitch and roll control by effectively shifting the center of weight of the aircraft with respect to the thrust vector of the coaxial rotor set. Such a system is disclosed, for example, in U.S. Pat. No. 5,791,592 to Nolan, et al. (1998). In this simplified system, there is no need for cyclic blade pitch control, and there is no collective pitch control. Tilt of the coaxial rotor set, and increasing or decreasing the speed of the rotors, provides pitch, roll and lift control. Since, as mentioned, the disk loading in coaxial helicopters is higher, and rotor diameter is smaller than conventional designs, adequate control of lift is possible without collective blade pitch control, though some lag in response is deemed inherent, and should be taken into account by a pilot operating a helicopter of this design.
Yaw control in the Nolan device is by means of two sets of airfoils which are tiltable. The airfoil sets tilt with respect to two sets of axes. One set of axes is roughly parallel, and the other is normal, respectively, to the rotor thrust vector when the airfoils are vertically oriented. A larger airfoil set rotates about axes normal to the thrust vector, and impinges on the downwash from the rotor set. As the airfoils tilt to the right or left from a roughly vertical neutral orientation, this creates a reaction force vector tending to yaw the airframe right or left, depending on the angular direction of tilt of the larger set of airfoils. The second set of airfoils, which are smaller, and depend rudder-like from a rear edge of the larger airfoils, turn back and forth about the axis parallel to the thrust vector when the larger airfoils are upright. The second set of airfoils appear to function in a manner similar to a tail rudder in a conventional aircraft, and therefore appear from the disclosure to be more effective in yaw control when the device has developed significant forward speed, and to be less effective in yaw control when the helicopter is hovering at a stationary point, or otherwise has very low forward speed.
With this background, it has been recognized by the inventors that for all the potential advantages of coaxial designs, heretofore there has not been developed a coaxial rotor aircraft in the ultralight class (as defined by FAA regulations e.g. 14 C.F.R. §103) which provides acceptable flight characteristics at low cost. Known ultralight helicopters are of single-rotor design. Such known ultralight helicopters essentially mimic full-size conventional helicopter propulsion and control systems, and tend to be expensive.